Rhabdomyosarcoma, one of the most common solid tumors of childhood, is believed to arise as a result of a regulatory abnormality of the growth and differentiation of myogenic precursor cells. Despite the expression of myogenic basic-helix-loop-helix proteins of the MyoD family, rhabdomyosarcoma cells fail to withdraw from the cell cycle and are unable to differentiate into muscle cells. The OBJECTIVE of this research proposal is to investigate the molecular mechanisms that prevent differentiation in human embryonal rhabdomyosarcoma (RD) cells. By applying combined molecular and biochemical approaches, we hope to understand how regulatory defects could occur at multiple steps of the muscle differentiation program and find clues to the origin of rhabdomyosarcomas. The SPECIFIC AIMS are: (I) Identify the factors that prevent MyoD from activating a subset of promoters in RD cells using chromatin immuno-precipitation assays; (II) Characterize post-translational modifications and their functionality on E2A proteins using mutagenesis and protein-protein interaction assays; (III) Identify and characterize RD-specific alterations in the MyoD- associated transcriptional complex by Tandem Affinity Purification and tandem mass spectrometry.